Method of forming a non-volatile resistance variable device, and non-volatile resistance variable device

ABSTRACT

A method of metal doping a chalcogenide material includes forming a metal over a substrate. A chalcogenide material is formed on the metal. Irradiating is conducted through the chalcogenide material to the metal effective to break a chalcogenide bond of the chalcogenide material at an interface of the metal and chalcogenide material and diffuse at least some of the metal outwardly into the chalcogenide material. A method of metal doping a chalcogenide material includes surrounding exposed outer surfaces of a projecting metal mass with chalcogenide material. Irradiating is conducted through the chalcogenide material to the projecting metal mass effective to break a chalcogenide bond of the chalcogenide material at an interface of the projecting metal mass outer surfaces and diffuse at least some of the projecting metal mass outwardly into the chalcogenide material. In certain aspects, the above implementations are incorporated in methods of forming non-volatile resistance variable devices. In one implementation, a non-volatile resistance variable device in a highest resistance state for a given ambient temperature and pressure includes a resistance variable chalcogenide material having metal ions diffused therein. Opposing first and second electrodes are received operatively proximate the resistance variable chalcogenide material. At least one of the electrodes has a conductive projection extending into the resistance variable chalcogenide material.

TECHNICAL FIELD

[0001] This invention relates to non-volatile resistance variabledevices and methods of forming the same.

BACKGROUND OF THE INVENTION

[0002] Semiconductor fabrication continues to strive to make individualelectronic components smaller and smaller, resulting in ever denserintegrated circuitry. One type of integrated circuitry comprises memorycircuitry where information is stored in the form of binary data. Thecircuitry can be fabricated such that the data is volatile ornon-volatile. Volatile storing memory devices result in loss of datawhen power is interrupted. Non-volatile memory circuitry retains thestored data even when power is interrupted.

[0003] This invention was principally motivated in making improvementsto the design and operation of memory circuitry disclosed in the Kozickiet al. U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796,which ultimately resulted from U.S. patent application Ser. No.08/652,706, filed on May 30, 1996, disclosing what is referred to as aprogrammable metalization cell. Such a cell includes opposing electrodeshaving an insulating dielectric material received therebetween. Receivedwithin the dielectric material is a fast ion conductor material. Theresistance of such material can be changed between highly insulative andhighly conductive states. In its normal high resistive state, to performa write operation, a voltage potential is applied to a certain one ofthe electrodes, with the other of the electrode being held at zerovoltage or ground. The electrode having the voltage applied theretofunctions as an anode, while the electrode held at zero or groundfunctions as a cathode. The nature of the fast ion conductor material issuch that it undergoes a chemical and structural change at a certainapplied voltage. Specifically, at some suitable threshold voltage,plating of metal from metal ions within the material begins to occur onthe cathode and grows or progresses through the fast ion conductortoward the other anode electrode. With such voltage continued to beapplied, the process continues until a single conductive dendrite orfilament extends between the electrodes, effectively interconnecting thetop and bottom electrodes to electrically short them together.

[0004] Once this occurs, dendrite growth stops, and is retained when thevoltage potentials are removed. Such can effectively result in theresistance of the mass of fast ion conductor material between electrodesdropping by a factor of 1,000. Such material can be returned to itshighly resistive state by reversing the voltage potential between theanode and cathode, whereby the filament disappears. Again, the highlyresistive state is maintained once the reverse voltage potentials areremoved. Accordingly, such a device can, for example, function as aprogrammable memory cell of memory circuitry.

[0005] The preferred resistance variable material received between theelectrodes typically and preferably comprises a chalcogenide materialhaving metal ions diffused therein. A specific example is germaniumselenide with silver ions. The present method of providing the silverions within the germanium selenide material is to initially deposit thegermanium selenide glass without any silver being received therein. Athin layer of silver is thereafter deposited upon the glass, for exampleby physical vapor deposition or other technique. An exemplary thicknessis 200 Angstroms or less. The layer of silver is irradiated, preferablywith electromagnetic energy at a wavelength less than 500 nanometers.The thin nature of the deposited silver enables such energy to passthrough the silver to the silver/glass interface effective to break achalcogenide bond of the chalcogenide material, thereby effectingdissolution of silver into the germanium selenide glass. The appliedenergy and overlying silver result in the silver migrating into theglass layer such that a homogenous distribution of silver throughout thelayer is ultimately achieved.

[0006] It can be challenging to etch and to chemical-mechanical polishmetal ion containing chalcogenide materials. Accordingly it would bedesirable to develop memory cell fabrication methods which avoid one orboth of etching or polishing such materials. It would also be desirableto develop alternate methods from that just described which incorporatethe metal ions into chalcogenide materials. While the invention wasprincipally motivated in achieving objectives such as these, theinvention is in no way so limited. The artisan will appreciateapplicability of the invention in other aspects of processing involvingchalcogenide materials, with the invention only being limited by theaccompanying claims as literally worded and as appropriately interpretedin accordance with the doctrine of equivalents.

SUMMARY

[0007] The invention includes non-volatile resistance variable devicesand methods of forming the same. In one implementation, a method ofmetal doping a chalcogenide material includes forming a metal over asubstrate. A chalcogenide material is formed on the metal. Irradiatingis conducted through the chalcogenide material to the metal effective tobreak a chalcogenide bond of the chalcogenide material at an interfaceof the metal and chalcogenide material and diffuse at least some of themetal outwardly into the chalcogenide material. In one implementation, amethod of metal doping a chalcogenide material includes surroundingexposed outer surfaces of a projecting metal mass with chalcogenidematerial. Irradiating is conducted through the chalcogenide material tothe projecting metal mass effective to break a chalcogenide bond of thechalcogenide material at an interface of the projecting metal mass outersurfaces and diffuse at least some of the projecting metal massoutwardly into the chalcogenide material. In certain aspects, the aboveimplementations are incorporated in methods of forming non-volatileresistance variable devices.

[0008] In one implementation, a non-volatile resistance variable devicein a highest resistance state for a given ambient temperature andpressure includes a resistance variable chalcogenide material havingmetal ions diffused therein. Opposing first and second electrodes arereceived operatively proximate the resistance variable chalcogenidematerial. At least one of the electrodes has a conductive projectionextending into the resistance variable chalcogenide material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

[0010]FIG. 1 is a diagrammatic sectional view of a semiconductor waferfragment in process in accordance with an aspect of the invention.

[0011]FIG. 2 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 1.

[0012]FIG. 3 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 2.

[0013]FIG. 4 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 3.

[0014]FIG. 5 is a view of the FIG. 1 wafer fragment at an alternateprocessing step subsequent to that shown by FIG. 3.

[0015]FIG. 6 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 4.

[0016]FIG. 7 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 6.

[0017]FIG. 8 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] This disclosure of the invention is submitted in furtherance ofthe constitutional purposes of the U.S. Patent Laws “to promote theprogress of science and useful arts” (Article 1, Section 8).

[0019] Referring to FIG. 1, a semiconductor wafer fragment 10 is shownin but one preferred embodiment of a method of forming a non-volatileresistance variable device. By way of example only, example such devicesinclude programmable metalization cells and programmable opticalelements of the patents referred to above, further by way of exampleonly, including programmable capacitance elements, programmableresistance elements, programmable antifuses of integrated circuitry andprogrammable memory cells of memory circuitry. The above patents areherein incorporated by reference. The invention contemplates thefabrication techniques and structure of any existing non-volatileresistance variable device, as well as yet-to-be developed such devices.In the context of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. Also in the context of this document, the term “layer”encompasses both the singular and the plural unless otherwise indicated.Further, it will be appreciated by the artisan that “resistance setablesemiconductive material” and “resistance variable device” includesmaterials and devices wherein a property or properties in addition toresistance is/are also varied. For example, and by way of example only,the material's capacitance and/or inductance might also be changed inaddition to resistance.

[0020] Semiconductor wafer fragment 10 comprises a bulk monocrystallinesemiconductive material 12, for example silicon, having an insulativedielectric layer 14, for example silicon dioxide, formed thereover. Aconductive electrode material 16, also termed a first metal layer, isformed over and on dielectric layer 14. By way of example only,preferred materials include any of those described in the incorporatedKozicki et al. patents referred to above in conjunction with thepreferred type of device being fabricated. Layer 16 might constitute apatterned electrode for the preferred non-volatile resistance variabledevice being fabricated. Alternately by way of example only, layer 16might constitute a patterned line or extension of a field effecttransistor gate, with a subsequently deposited layer principally servingessentially as the electrode. An example preferred material for layer 16is elemental tungsten deposited to an exemplary thickness of from about100 Angstroms to about 1000 Angstroms. In the illustrated example, layer16 has been patterned, and another dielectric layer 17 has beendeposited and planarized as shown.

[0021] A second metal layer 18 is formed (preferably by a blanketdeposition) on first metal layer 16. An exemplary preferred material inconjunction with the non-volatile resistance variable device beingfabricated is elemental silver. A preferred thickness for layer 18 isfrom about 175 Angstroms to about 300 Angstroms.

[0022] Referring to FIG. 2, second metal layer 18 is formed into astructure 20, and first metal layer 16 is outwardly exposed. Such ispreferably conducted by subtractive patterning of metal layer 18, forexample by photolithographic patterning and etch. In one implementation,structure 20 can be considered as comprising a metal mass projectingfrom underlying substrate material and having outer surfaces comprisedof a top surface 22 and opposing side surfaces 24 which join with topsurface 22 at respective angles. The preferred angles are preferablywithin about 15° of normal, with normal angles being shown in thefigures.

[0023] Referring to FIG. 3, a chalcogenide material 26 is formed overthe substrate on second metal structure 20 outer surfaces 22 and 24, andon exposed first metal layer 16. Such is preferably formed by blanketphysical vapor deposition. A preferred deposition thickness for layer 26is preferably less than three times the thickness of deposited layer 18,with an example being from about 525 Angstroms to about 900 Angstroms.More preferred is a thickness to provide layer 18 at 20% to 50% of layer26 thickness. Exemplary preferred chalcogenide materials include thosedisclosed in the Kozicki et al. patents referred to above. Specificpreferred examples include a chalcogenide material having metal ionsdiffused therein represented by the formula Ge_(x)A_(y), where “A” isselected from the group consisting of Se, Te and S and mixtures thereof.The illustrated example provides but one possible example of surroundingthe exposed outer surfaces of a projecting metal mass with chalcogenidematerial in accordance with but one aspect of the invention.

[0024] Referring to FIG. 4, irradiating is conducted throughchalcogenide material 26 to patterned second metal 18 effective to breaka chalcogenide bond of the chalcogenide material at an interface withthe patterned second metal outer surfaces and the chalcogenide material,and to diffuse at least some of second metal 18 outwardly into thechalcogenide material. Metal doped material 27 is formed thereby.Therefore as shown in the preferred embodiment, only a portion ofblanket deposited chalcogenide material layer 26 is doped with secondmetal 18. A preferred irradiating includes exposure to actinic radiationhaving a wavelength below 500 nanometers, with radiation exposure atbetween 404-408 nanometers being a more specific example. A specificexample in a suitable UV radiation flood exposure tool is 4.5 mW/cm², 15minutes, 405 nm wavelength, at room ambient temperature and pressure.

[0025] In the depicted and preferred embodiment, the irradiatingdiffuses only some of the metal from layer 18 outwardly intochalcogenide material, leaving a remnant structure 20 a. Accordingly,the projecting metal mass 20 a has a shape after the irradiating whichis maintained in comparison to original shape 20, but at a reduced size.FIG. 5 illustrates a lesser preferred alternate embodiment 10 a wherebythe irradiating and/or layer dimensions might be modified such that theirradiating diffuses all of projecting metal mass 20 outwardly into thechalcogenide material.

[0026] The preferred exemplary tungsten material of layer 16 does notappreciably diffuse into layer 26. Referring to FIG. 6 and regardless,chalcogenide material 26 not doped with metal 18 is substantiallyselectively etched from metal doped portion 27 of the chalcogenidematerial. In the context of this document, “substantially selective”means a relative etch ratio of layer 26 relative to layer 27 or at least3:1. In the illustrated and preferred embodiment, such etching ispreferably conducted to remove all of chalcogenide material 26 which hasnot been doped with metal 18. The preferred etching comprises dryanisotropic etching, preferably dry plasma anisotropic etching. Aprinciple preferred component of such etching gas comprises CF₄.Additional preferred gases in the chemistry include C₂F₆ and C₄F₈. Toppower is preferably maintained at 500 watts, with the lower wafersusceptor being allowed to float. Susceptor temperature is preferablymaintained at about 25° C., and an exemplary reactor pressure is 50mTorr. By way of example only, a specific example in a reactive ionetcher is CF₄ at 50 sccm, Ar at 25 sccm, susceptor temperature at 25°C., pressure of 50 mTorr and top power at 500 Watts.

[0027] Referring to FIG. 7, an insulating layer 30 has been depositedand metal doped chalcogenide material 27 has been exposed. An exampleand preferred material for layer 30 is silicon nitride.

[0028] Referring to FIG. 8, an outer conductive electrode layer 32 hasbeen deposited and patterned to form the outer electrode of thepreferred non-volatile resistance variable device. Example materialsinclude those disclosed in the above Kozicki et al. patents. In theillustrated and described preferred example, silver structure 20 a mightbe designed and fabricated to constitute the effective quantity ofsilver for programming the device with no silver being provided inelectrode 32. Alternately by way of example only, layer 16 might alsoconstitute elemental silver with no silver being provided in electrode32. Further, by way of example only, electrode 32 might principallycomprise elemental silver, or at least a lower silver portion in contactwith the chalcogenide material 27.

[0029] The above-described preferred embodiment example was inconjunction with fabrication of a non-volatile resistance variabledevice. However, the invention also contemplates metal doping achalcogenide material independent of the device being fabricated, and inthe context of the accompanying claims as literally worded regardingmethods of metal doping a chalcogenide material. Further, the preferredexample is with respect to formation of a projecting metal from anunderlying substrate having chalcogenide material received thereover.However, the invention is in no way so limited and also contemplates, byway of example only, diffusing metal from an entirely flat, or other,underlying surface into overlying chalcogenide material.

[0030] The invention also contemplates non-volatile resistance variabledevices independent of the method of manufacture. In one implementation,such a device includes a projecting metal mass (for example mass 20 a)extending outwardly from a first metal layer laterally central intoresistance variable chalcogenide material. In one aspect, the inventioncontemplates the device being in a highest resistance state for a givenambient temperature and pressure. For example, the FIG. 8 device asdepicted is in such a highest state of resistance. Progressively lowerstates of resistance for a given ambient temperature and pressure willexist as a silver dendrite, in the preferred embodiment, progressivelygrows from an electrode to the point of contacting the opposingelectrode. FIG. 8 depicts but one exemplary embodiment of such anon-volatile resistance variable device having such a laterally centrallocated projecting mass relative to material 27.

[0031] The invention also contemplates a non-volatile resistancevariable device in a highest resistance state for a given ambienttemperature and pressure independent of a conductive projection which isso centrally located. Such comprises a resistance variable chalcogenidematerial having metal ions diffused therein. Opposing first and secondelectrodes are received operatively proximate the resistance variable,chalcogenide material, with at least one of the electrodes comprising aconductive projection extending into the resistance variablechalcogenide material. Provision of such a structure is in no way shownor suggested in a highest resistance state for a given ambienttemperature and pressure in any of the teachings and drawings of theabove-described Kozicki et al. patents.

[0032] In compliance with the statute, the invention has been describedin language more or less specific as to structural and methodicalfeatures. It is to be understood, however, that the invention is notlimited to the specific features shown and described, since the meansherein disclosed comprise preferred forms of putting the invention intoeffect. The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of metal doping a chalcogenide material comprising: forminga metal over a substrate; forming a chalcogenide material on the metal;and irradiating through the chalcogenide material to the metal effectiveto break a chalcogenide bond of the chalcogenide material at aninterface of the metal and chalcogenide material and diffuse at leastsome of the metal outwardly into the chalcogenide material.
 2. Themethod of claim 1 wherein the metal comprises elemental silver.
 3. Themethod of claim 1 wherein the chalcogenide material having metal ionsdiffused therein comprises Ge_(x)A_(y), where A is selected from thegroup consisting of Se, Te and S, and mixtures thereof.
 4. The method ofclaim 1 wherein the irradiating diffuses only some of the metaloutwardly into the chalcogenide material.
 5. The method of claim 1wherein the irradiating diffuses all of the metal outwardly into thechalcogenide material.
 6. A method of metal doping a chalcogenidematerial comprising: surrounding exposed outer surfaces of a projectingmetal mass with chalcogenide material; and irradiating through thechalcogenide material to the projecting metal mass effective to break achalcogenide bond of the chalcogenide material at an interface of theprojecting metal mass outer surfaces and diffuse at least some of theprojecting metal mass outwardly into the chalcogenide material.
 7. Themethod of claim 6 wherein the projecting mass and outer surfacescomprises a top surface joined with opposing side surfaces at respectiveangles.
 8. The method of claim 6 wherein the projecting mass and outersurfaces comprises a top surface joined with opposing side surfaces atrespective angles within about 15° degrees of normal.
 9. The method ofclaim 6 wherein the irradiating diffuses only some of the projectingmetal mass outwardly into the chalcogenide material.
 10. The method ofclaim 6 wherein the irradiating diffuses all of the projecting metalmass outwardly into the chalcogenide material.
 11. The method of claim 6wherein the projecting metal mass has a shape which is maintained afterthe irradiating but at a reduced size.
 12. The method of claim 6 whereinthe surrounding comprises blanket depositing of the chalcogenidematerial.
 13. A method of metal doping a chalcogenide materialcomprising: forming a metal over a substrate; patterning the metal intoa structure having an outer surface; forming a chalcogenide material onthe metal structure outer surface; and irradiating through thechalcogenide material to the patterned metal effective to break achalcogenide bond of the chalcogenide material at an interface of thepatterned metal outer surface and the chalcogenide material and diffuseat least some of the metal outwardly into the chalcogenide material. 14.The method of claim 13 wherein the patterning is subtractive of themetal.
 15. The method of claim 13 wherein the patterning comprisesphotolithography.
 16. The method of claim 13 wherein the irradiatingdiffuses only some of the metal outwardly into the chalcogenidematerial.
 17. The method of claim 13 wherein the irradiating diffusesall of the metal outwardly into the chalcogenide material.
 18. Themethod of claim 13 wherein the structure has a shape which is maintainedafter the irradiating but at a reduced size.
 19. A method of metaldoping a chalcogenide material comprising: forming a metal over asubstrate; patterning the metal into a structure having an outersurface; blanket depositing a chalcogenide material over the substrateand on the metal structure outer surface; irradiating through thechalcogenide material to the patterned metal effective to break achalcogenide bond of the chalcogenide material at an interface of thepatterned metal outer surface and the chalcogenide material and diffuseat least some of the metal outwardly into the chalcogenide material, andthereby metal doping only a portion of the blanket depositedchalcogenide material; and substantially selectively etchingchalcogenide material not doped with the metal from the metal dopedportion of the chalcogenide material.
 20. The method of claim 19 whereinthe depositing comprises chemical vapor deposition.
 21. The method ofclaim 19 wherein the forming comprises a blanket deposition.
 22. Themethod of claim 19 wherein the etching comprises dry anisotropicetching.
 23. The method of claim 19 wherein the etching comprises dryanisotropic etching using a gas chemistry comprising CF₄.
 24. The methodof claim 19 wherein the irradiating diffuses only some of the metaloutwardly into the chalcogenide material.
 25. The method of claim 19wherein the irradiating diffuses all of the metal outwardly into thechalcogenide material.
 26. The method of claim 19 wherein the structurehas a shape which is maintained after the irradiating but at a reducedsize.
 27. The method of claim 19 wherein the etching removes allchalcogenide material not doped with the metal from the substrate.
 28. Amethod of forming a non-volatile resistance variable device, comprising:surrounding exposed outer surfaces of a projecting metal mass withchalcogenide material; irradiating through the chalcogenide material tothe projecting metal mass effective to break a chalcogenide bond of thechalcogenide material at an interface of the projecting metal mass outersurfaces and diffuse at least some of the projecting metal massoutwardly into the chalcogenide material; and after the irradiatingforming an outer electrode over the chalcogenide material.
 29. Themethod of claim 28 wherein the projecting metal mass and outer surfacescomprises a top surface joined with opposing side surfaces at respectiveangles.
 30. The method of claim 28 wherein the projecting metal mass andouter surfaces comprises a top surface joined with opposing sidesurfaces at respective angles within about 15° degrees of normal. 31.The method of claim 28 wherein the projecting metal mass has a shapewhich is maintained after the irradiating but at a reduced size.
 32. Themethod of claim 28 wherein the surrounding comprises blanket depositingof the chalcogenide material.
 33. A method of forming a non-volatileresistance variable device, comprising: forming a first metal layer overa substrate; forming a second metal layer on the first metal layer;patterning the second metal layer into a structure having an outersurface, and exposing the first metal layer; blanket depositing achalcogenide material over the substrate on the second metal structureouter surface and on the exposed first metal layer; irradiating throughthe chalcogenide material to the patterned second metal effective tobreak a chalcogenide; bond of the chalcogenide material at an interfaceof the patterned second metal outer surface and the chalcogenidematerial and diffuse at least some of the second metal outwardly intothe chalcogenide material, and thereby second metal doping only aportion of the blanket deposited chalcogenide material; substantiallyselectively etching chalcogenide material not doped with the secondmetal from the second metal doped portion of the chalcogenide material;and after the etching, forming an outer electrode over the chalcogenidematerial.
 34. The method of claim 33 wherein the etching comprises dryanisotropic etching.
 35. The method of claim 33 wherein the etchingcomprises dry anisotropic etching using a gas chemistry comprising CF₄.36. The method of claim 33 wherein the irradiating diffuses only some ofthe metal outwardly into the chalcogenide material.
 37. The method ofclaim 33 wherein the structure has a shape which is maintained after theirradiating but at a reduced size.
 38. A non-volatile resistancevariable device, comprising: a substrate comprising a first metal layer;an insulative layer received over the first metal layer; a resistancevariable chalcogenide material having metal ions diffused thereinreceived within an opening formed through the insulative layer; aprojecting metal mass extending outwardly from the first metal layerlaterally central into the resistance variable chalcogenide material;and an electrode spaced from the projecting metal mass and first metallayer operatively adjacent the resistance variable chalcogenidematerial.
 39. The device of claim 38 in a highest resistance state for agiven ambient temperature and pressure.
 40. The device of claim 38wherein the projecting metal mass comprises a top surface joined withopposing side surfaces at respective angles.
 41. The device of claim 38wherein the projecting metal mass comprises a top surface joined withopposing side surfaces at respective angles within about 15° degrees ofnormal.
 42. The device of claim 38 wherein the metal mass compriseselemental silver.
 43. The device of claim 38 wherein the chalcogenidematerial having metal ions diffused therein comprises Ge_(x)A_(y), whereA is selected from the group consisting of Se, Te and S, and mixturesthereof.
 44. A non-volatile resistance variable device in a highestresistance state for a given ambient temperature and pressure,comprising: a resistance variable chalcogenide material having metalions diffused therein; and opposing first and second electrodes receivedoperatively proximate the resistance variable chalcogenide material, atleast one of the electrodes comprising a conductive projection extendinginto the resistance variable chalcogenide material.
 45. The device ofclaim 44 wherein the conductive projection comprises a top surfacejoined with opposing side surfaces at respective angles.
 46. The deviceof claim 44 wherein the conductive projection comprises a top surfacejoined with opposing side surfaces at respective angles within about 15°degrees of normal.
 47. The device of claim 44 wherein the one electrodeand the conductive projection comprise the same material.
 48. The deviceof claim 44 wherein the one electrode and the conductive projectioncomprise elemental, silver.
 49. The device of claim 44 wherein theconductive projection comprises elemental silver.
 50. The device ofclaim 44 wherein the chalcogenide material having metal ions diffusedtherein comprises Ge_(x)A_(y), where A is selected from the groupconsisting of Se, Te and S, and mixtures thereof.